KR0145356B1 - Apparatus and method for supplying welding current from a brushless alternator - Google Patents

Abstract

The present invention relates to a welding power supply having a thyristor rectifier and a polyphase synchronous generator using a control circuit to selectively prevent firing of the thyristor when a short circuit occurs. According to the invention a reliable operation of the synchronous generator welding power supply under short circuit conditions is achieved.

Description

Apparatus and method for supplying welding current from a brushless alternator

1 is a schematic diagram illustrating an engine driven welder including an alternator, rectifier and control circuitry.

2 is a flow chart illustrating some of the control sequences and mechanisms used in the circuit of FIG.

3 is a flow diagram for a zero-crossing interrupt for obtaining time information of the FIG. 1 circuit.

4 is a flow diagram for a feedback and control interrupt sequence that implements the main control functions of the FIG. 1 circuit.

5 is a schematic diagram showing the winding of the alternator shown in FIG.

6 is a schematic diagram of a voltage sensing circuit used in the circuit of FIG.

FIG. 7 shows a preferred short circuit firing sequence achieved by the circuit of FIG.

8 is an output current waveform diagram obtained in the circuit of FIG.

9 is an output volt-ampere curve diagram obtained in the circuit of FIG.

* Explanation of symbols for main parts of the drawings

14: alternator 30: rectifier bridge

130: digital processing circuit 142: voltage sensing circuit

144: potentiometer 146: rectifier

149: threshold detector

The present invention relates to an engine driven welding power supply, and more particularly to a DC welding power supply for supplying current to a polyphase thyristor rectifier circuit using an engine driven polyphase alternator. The present invention particularly provides a method for supplying a short circuit state current from a polyphase alternator to a welding device.

As a welding power supply, a welding power supply including gasoline or diesel fuel is generally used. Such products typically include gasoline or diesel engines for driving generators that supply the electrical outputs used to generate arc and welding voltages. Occasionally, three-phase alternators are used, and three-phase brushless induction alternators are known. Such alternators are usually cheaper, stronger and more reliable than other types of manufacture. The disadvantage of brushless alternators occurs when their outputs are short circuited. In the short-circuit state, the current flows at all or very irregularly. In most cases, no short circuit condition occurs and the problem as described above is not critical. In welding, however, a short circuit condition or a condition very similar to the short circuit condition often occurs. Therefore, lack of output or irregular operation while in short circuit condition is an important issue.

The present invention provides a method of controlling the output of a three-phase rectified brushless alternator that supplies a control current even when the output is in a short circuit state or the state approaches a short circuit. According to a first aspect of the present invention, a multiphase induction alternator supplies a three phase current to a three phase thyristor rectifier for supplying a DC output current for welding. The means for detecting the magnitude of the weld output detects the presence of a short circuit condition and prevents selected thyristors of the thyristors of the rectifier circuit from being controlled during firing of at least one of the remaining thyristors. Supply the output.

According to a second aspect of the invention there is provided a sensing means for detecting a short circuit condition by sensing a winding output voltage and detecting an average voltage drop over a predetermined value.

According to a third aspect of the invention, microprocessor control is used to control the magnitude of the arc current in accordance with a user-selected parameter and to selectively trigger or prevent the thyristors in response to a short circuit condition.

According to a fourth aspect of the present invention, a method for supplying current to an arc welder includes: providing a multiphase alternator having a polyphase output; Receiving a output from an alternator and providing a multiphase rectifier composed of several thyristors having a direct current output; Detecting the presence of a short circuit condition; Preventing the selected thyristor from conducting for the duration of the short circuit condition when a short circuit condition is detected.

It is a first object of the present invention to provide a multiphase brushless alternator welder with good short circuit response.

It is a second object of the present invention to provide a method of controlling a rectifier for a brushless alternator welder in which a short circuit current can be reasonably controlled.

It is a third object of the present invention to provide a method of controlling a rectifier for a brushless polyphase alternator which can be maintained while the output current is in a short circuit state.

It is a fourth object of the present invention to provide a controller for a rectified brushless polyphase alternator welding power supply that is reliable in operation, capable of adjusting short circuit conditions, and which is low in manufacturing cost.

The above and other objects of the present invention will become apparent from the following detailed description, the accompanying drawings and the claims.

A list of attached programs detailing part of the FIG. 4 method is appended as an appendix at the end of the present specification.

With reference to the accompanying drawings, which are not intended to limit the present invention but merely to describe preferred embodiments of the present invention, FIG. 1 shows an arc welding power supply, the engine 10 of which a shaft ( 12 is physically connected to the alternator 14. The alternator 14 is a three-phase inductive brushless alternator 14 having a three-phase welding power output section 16 and a synchronous output section 20. Typically such alternators may also have an auxiliary output for supplying 120V or 240V current. As shown in FIG. 5, the alternator has three delta connection welding current windings 24, 25 and 26 and two excitation windings 27 and 28. As shown in FIG. Capacitor 29 supplies reactive power when needed. Two auxiliary capacitors (not shown) typically interconnect three phases in a conventional manner.

6 shows the voltage sensing circuit 142 of FIG. 1 in more detail. The potentiometer 144 divides the high voltage at the output part A and supplies the divided voltage to the rectifier 146. The output of rectifier 146 is filtered by capacitor 148 and supplied to threshold detector 149. The threshold detector 149 supplies an output representing the output voltage at the output terminal A. The very low output maintained for the selected period indicates a short circuit condition. The filtering applied in the sense circuit 142 prevents the extremely short duration short circuit that may occur during transition welding from being interpreted as a harmful short circuit. The selection and filtering of appropriate constants, such as voltages that indicate short circuits that require corrective action, allows temporary short circuits that will not adversely affect the alternator.

The three-phase welding power output is provided on three alternator output lines 22A, 22B, 22C which supply three-phase output in a conventional manner. The rectifier bridge 30 receives the three-phase output of the output lines 22A, 22B, and 22C. The rectifier bridge 30 is composed of three semiconductor controlled rectifiers (SCRs) 31, 32, and 33 and three diodes 34, 35, and 36. These SCRs and diodes are connected by a conventional bridge connection method. Each SCR and diode receives a current from one of the three phase power lines and supplies this current to the DC welding power output line 40 and the DC welding ground line 42. Filtering elements such as inductors, chokes and / or capacitors are sometimes added in a conventional manner. The effect of the output choke 100 shown in FIG. 1 is shown in FIG. The upper curve shows the output current without choke. The lower curve shows the output current when the choke 100 is provided. The SCR-diode bridges described are well known in the art. It is also known to use six SCRs alternatively and available.

Capacitors (not shown) are typically connected between the extensions of these windings or the alternator output lines 22A, 22B, 22C to supply reactive power as needed.

The DC welding power output on the output line 40 is connected to the welding electrode 41 and the DC ground 41 is connected to the workpiece 43. Both of these are electrically connected by the arc 44 as is conventional. In addition, the polarity of the electrode and the ground can be changed as necessary. The arc current is sensed by the current sense classifier 46 and the current magnitude information is examined and passed to the digital processing circuit 130. The arc voltage sense line 48 may be used to supply information about the arc voltage to the digital processing circuit 130.

The predetermined power input unit 140 is provided on the welding power supply device. The predetermined current input unit 140 may take the form of dials, pushbuttons, or other conventional means capable of inputting digital information to an electrical device. This current input allows the operator to set the desired welding current and communicate this information to the digital processing circuit 130. This digital processing circuit 130 actually controls the current and achieves the desired result. Other welding parameters may also be set at the input 140. The digital processing circuit controls the firing of the SCRs 31, 32, 33 by supplying a gating signal to the gate terminals G1, G2, G3 of the SCR. As is conventional, rectifier bridge 30 receives three-phase power from alternator 14. The output current of the rectifier bridge 30 is controlled by turning on the SCRs 31, 32, 33 for the period selected by the signals on the gates G1, G2, G3. The digital processing circuit 130 receives information from the current sense classifier 46, the voltage line 48, and the synchronization line 20. The digital processing circuit 130 determines how advanced or delayed the gating signal is by comparing the actual current with a predetermined current set through the current input unit 140. The gating signal is synchronized by the synchronization information on the synchronization line 20. When more current is needed, the SCR is turned on ahead of 1/2 cycle of positive flow and more current is supplied to the arc 44.

If a short circuit condition occurs in the arc 44, virtually no impedance can be found at the output lines 22A, 22B, 22C of the alternator. Capacitors and / or inductors supplying reactive power and magnetizing current are short circuited. This causes the magnetic field to generate current in the output line to disintegrate and completely stop the output current and / or irregular operation. This problem is solved by the present invention by providing the alternator with several impedances during each cycle to maintain the magnetic field in the alternator in a reliable manner. If a short circuit condition is detected, no gating signal is supplied to one or more SCRs 31, 32, 33. The SCR cannot turn on unless it receives a gating signal and cannot conduct current. This provides an impedance to the alternator and prevents the collapse of the magnetic field. In a preferred embodiment these are achieved through software control. However, the same circuit conditions can be achieved by digital or analog electronic control.

Means for controlling the gating signal to ensure impedance are shown in FIGS. 2-7. The main program 150 (FIG. 2) includes an initialization step 152, an initial period setting step 154, a service indication and a switching step 156. Basically, the main program performs the device self-check when the device is turned on, calculates the oscillation period of the three-phase power as seen in the three-phase power output unit 16, and sets the front panel 140. Read user input. The remaining control functions are driven by interrupts. It must be remembered that the microprocessor controls the operation very quickly. Conventional microprocessors operate at clock frequencies of 2 MHz-100 MHz. Meanwhile, the frequency of a three-phase alternator is about 50 Hz or 60 Hz. Thus, the microprocessor executes thousands to millions of steps during each cycle of a three-phase alternator.

An important timing reference for control is the synchronization signal on the synchronization line 20. The synchronization signal is applied to a zero cross detector in the digital processing circuit 130. When a zero crossing is detected, a signal is provided to the microprocessor that sets the zero crossing flag. This is illustrated by a zero cross interupt sequence 160 (Figure 3). The function of this sequence is to record the time of zero crossing by an alternating signal for use in determining the firing angle of the SCRs 31, 32 and 33. When the AC signal line transmits a voltage signal indicating the magnetic field position of the alternator rotor, the zero crossing of this signal is a fixed time relationship with respect to the zero crossing of the power signal with respect to all three-phase output lines 22A, 22B, 22C of the output power. Has Furthermore, since the rotational speed of the alternator rotor is completely constant compared to the clock speed of the digital processing circuit 130 and the direction of rotation is known, a single synchronization signal on the synchronization line 20 may cause all three SCRs (31, 32). (33) is suitable for use in generating a gating signal.

The feedback and control interrupt sequence 170 (see FIG. 4) performs the primary timing and firing operations for the SCR. This sequence is called at regular and frequent intervals. As described above, the alternator output on the output lines 22A, 22B, 22C has a frequency of about 50 Hz or 60 Hz. This gives a period of about 17 milliseconds. The feedback and control interrupt sequence 170 is called frequently enough to provide a suitable sampling rate for signals of this frequency and period. After being invoked, the feedback and control interrupt sequence 170 first checks to see if the new zero-crossing flag is set by zero-crossing interrupt sequence 160. As a result, if the flag is set, the sequence is a new zero-crossing branch step. Branch to (172). Next, the digital processing circuit 130 calculates a period between the newly set zero crossing time and the previous zero crossing time to calculate a new period in the calculation step 174. The information already read from the feedback circuit is processed to obtain the welding parameters, reference angles and the next sample interval for use in subsequent calculations. Next, the average calculation step 176 proceeds. In this step, the average of the monitored parameters is taken and calculated. The processing circuitry erases the zero crossing flag set in the zero crossing interrupt sequence 160 in the flag erasing step 178. The time for the next sample interrupt sequence 170 for start is also set in the flag clear step 178. The main sequence of feedback and control interrupt sequence 170 returns to arc feedback read step 184.

If the new zero-crossing branch step 172 determines that the zero-crossing flag is not set, this branch step is not taken and the feedback and control interrupt 170 sets the next sample interrupt in the next interrupt setting step 182. Continue with the main sequence starting with. Once the interrupt is initiated to initiate the next feedback and control interrupt sequence 170, the sequence of reading the current arc feedback continues in arc feedback reading step 184. The generator output voltage is sensed by the voltage sensing circuit 142 and supplied to the digital processing circuit 130. The arc current is sensed through the current sense classifier 46 and digitalized in the digital processing circuit 130. These values are read in the feedback reading step 184 and stored in memory for use in the average obtaining step 176 or to calculate the average when needed.

The average value calculated in step 176 and the feedback information obtained in step 184 are used to calculate the firing angle for the SCR in calculation step 186. As is generally known, SCRs become conductive only after the SCRs are forward biased and receive a gating pulse at their gate terminal. Therefore, the amount of current flowing through the SCR depends on the time point of the forward biased 1/2 cycle of the SCR when a gating pulse is received. The point where the gating pulse is applied to the gate terminal of the SCR is normally a reference for the angle measurement. In the present invention, the firing angle is calculated for each of the three SCRs. Next, the delay time from the zero crossing of the synchronization signal on each SCR line 20 is calculated and stored. The digital processing circuit 130 has information about each SCR when firing. In timing step 188, the feedback and control interrupt sequence compares the time set to fire the individual SCR with the current time from the clock to determine whether it is time to fire the SCR. If it is not time to fire the SCR, the sequence skips to the control input read and set point calculation step 210. If it is time to fire the SCR, a short circuit determination step 190 is executed. This step begins the sequence of determining whether the SCR call should be skipped to prevent the collapse of the magnetic field due to short circuit conditions. The digital processing circuit 130 compares the variable ARCV, which is a digital representation of the voltage sensed by the voltage sensing circuit 142, with the constant SHORT_VAL, which is a voltage value representing a short circuit. This comparison determines whether a short circuit condition exists. If there is no short circuit condition, the various count values used in the short circuit sequence are reset to zero in the counter and flag clear step 192, and the sequence is set to proceed to firing step 194 to call the next SCR. If the decision step 190 determines that a short circuit condition exists, then the count comparison decision step 196 is executed. The result achieved by the count comparison determining step is to fire a fixed number of SCRs (example 1) after detection of a short circuit and then de-curate a fixed number of SCRs (example 3).

With the name SHORT_CNT since the first detection of a short circuit condition, the number of consecutive SCR call counts is counted. The count comparison step 196 compares SHORT_CNT and SHORT_MAX. SHORT_MAX is the number of consecutive SCR calls required in a short circuit condition. If both are not the same no decision is made and the SHORT_CNT count is incremented at counter increment step 198. The sequence then returns to SCR firing step 194. If the SHORT_CNT count is equal to SHORT_MAX, then " YES " in step 196, and the sequence proceeds to a short coefficient comparison step 202.

The name of SHORT_CNT1 counts the number of consecutive SCR non-joints. SHORT_CNT1 is compared with the constant identified as SHORT_FACTOR in comparison step 202. SHORT_FACTOR is the number of consecutive skipped calls required during a short circuit condition. If both are the same, this means that a sufficient number of skipped firings of the SCR is achieved so that the erase step 204 is executed. Erasing step 204 resets SHORT_FLG and SHORT_CNT1 to zero to return to SCR firing step 194. However, if SHORT_CNT1 is not determined to be equal to SHORT_FACTOR in step 202, then the decision returned is no and the short count increment step 206 is executed. This prevents the digital processing circuit 130 from sending a gating pulse to the next scheduled SCR to achieve skip in the firing cycle. The branch of feedback and control sequence 170 then returns to the main sequence in control input read step 210. By selecting an appropriate constant value and increasing the count described above, any sequence of SCR firings can be easily skipped in response to a short circuit. A preferred sequence of callouts 1 and 3 is shown in FIG. This sequence places a short circuit current load on all SCRs. Any other suitable sequence for maintaining the magnetic field in the alternator is easily programmed.

The sequence of firing 1, skip 3 causes a large impedance supply to the alternator output. The output voltage will quickly build up so that the voltage sensing circuit 142 will indicate no short circuit and normal firing will be restored. If the short circuit condition persists, the above-described processing will be repeated immediately. This control sequence generates an arc volt-amperes curve as shown in FIG. The falling of the volt-ampere curve segment 222 results in non-short circuit control. The short-circuit firing skip sequence generally produces a linear low voltage portion 224.

Control input reading and setpoint calculation steps 210 are executed regardless of the output of decision steps 188,190,196,202. The settings on the front panel are read by the set point calculated for the microprocessor and the next interrupt sequence. The feedback and control interrupt sequence 170 is then terminated, and control of the digital processing circuit 130 is resumed until the interrupt again initiates the zero-crossing interrupt sequence 160 or the feedback control interrupt sequence 170. Return to 150). The sequence of instructions described above with reference to steps 190-206 is described in detail in the appendix of the program list that performs this control function.

The above described procedures and apparatus provide a mechanism for maintaining magnetization of the alternator during short circuit conditions that often occur during welding. This sequence is intended to provide reliable operation of a welding power supply using a brushless alternator as the power source. The mechanisms and methods described above are implemented in a microprocessor and the same mechanism can be implemented in discrete logic circuits or analog circuits.

It will be apparent to those skilled in the art that modifications and variations of the present invention can be made easily through the present specification. All such modifications and variations are included in the scope of the present invention as long as they are within the scope of the appended claims.

Claims (8)

An alternator 14 having a polyphase output; A rectifier bridge (30) comprising a plurality of thyristors (31, 32, 33) for receiving a multiphase output of said alternator and having a direct current output suitable for direct current arc welding; Means (142) for detecting the direct current output; Selector means (140) for causing an operator to select a predetermined value for the direct current output; Control means for controlling the operation of the thyristor and the direct current output in response to the selector means; Responsive to the short-circuit state for the polyphase output, comprising response means 130 such that while in the short-circuit state one or more thyristors of the thyristors remain non-conductive for one or more firing cycles. Power supply. 2. The apparatus according to claim 1, wherein said response means (130) comprises first counting means for counting thyristors number of times during a short circuit condition, and only one of said thyristors after said first counting means reaches a predetermined value. And the above thyristor maintains non-conductivity during the firing cycle of one ashing. 3. The apparatus of claim 2, wherein said response means (130) comprises second count means for counting a continuous time for which any thyristor maintains non-conductivity while in a short circuit condition, wherein the count value of said second count means is small. At least one of the thyristors maintains non-conductivity for a predetermined number of cycles when kept below stationary. 2. The apparatus of claim 1, wherein the means for detecting a direct current output (142) comprises means for detecting an alternator output voltage, and the response means (130) is a small indicative of the sensed alternator output voltage and short circuit condition. Welding power supply comprising means for comparing stationary conditions. Providing a polyphase induction alternator having a polyphase output; A multiphase rectifier receiving said polyphase output from said alternator, said rectifier output comprising a plurality of thyristors, each thyristor having a control gate and conducting a current in response to a signal on said control gate; Providing a; Providing means for sensing the magnitude of the alternator output; And an anti-conducting step such that the thyristors selected from the thyristors are not electrically conductive when the sensing means detects the magnitude of the alternator output indicating a short circuit condition. 6. The method of claim 5, wherein the anti-conducting step determines whether a short circuit exists, and if the short circuit does not exist, calls the next thyristor, and if the short circuit exists, counts the number of thyristor firings after the start of the short circuit state. Determining whether the number of firing numbers is equal to the predetermined number of firing numbers; If the number of firings is not equal to the predetermined number of firings, the next thyristor is fired. If the number of firings is the same as the predetermined number of firings, the number of thyristor firings which are sequentially blocked is counted. Determining whether they are the same; Firing the next thyristor if the number of stops is equal to the skip number, and preventing the next thyristor from firing if it is not equal. 6. The method of claim 5 wherein the rectifier consists of three semiconductor controlled rectifiers and three diodes. A method of supplying a welding current from a welding power supply comprising a rectifier bridge comprising a plurality of thyristors and an alternator, the method comprising: sensing an output voltage of the alternator; Comparing the sensed output voltage with a reference voltage to determine whether a short circuit exists; Skipping the call of a given thyristor of the bridge if a short circuit is present.